FIG. 1 shows the general construction of an optical fiber producing apparatus. FIG. 2 is a sectional view of a coated optical fiber 200C produced by the optical fiber producing apparatus.
An optical fiber preform 102 is heated in a drawing furnace 101 to produce an optical fiber 200 comprised of a core 201 of a diameter of about 10 .mu.m, in a case of a single mode optical fiber, and a cladding 202 of a diameter of 125 .mu.m formed on the core 201. The outer diameter of the optical fiber 200 is measured by an outer diameter measuring unit 104. Preferably, to protect the optical fiber 200 and improve its water resistance, it is treated with acetylene gas (C.sub.2 H.sub.2), chlorine gas (Cl.sub.2), etc. in a hot CVD reaction furnace to deposit a carbon hermetic coating 203 on the cladding 202 of the optical fiber 200 by CVD. Further, a primary resin coating 204 is given on the carbon hermetic coating 203 in a primary resin coating apparatus 106 comprised of a resin tank 107 and an ultraviolet curing apparatus 108. A secondary resin coating 205 is then given in a secondary resin coating apparatus 110 comprised of a resin coater 111 and an ultraviolet curing apparatus 112. Usually, the Young's modulus of the primary resin coating 204 is lower than the Young's modulus of the secondary resin coating 205, and the primary resin coating 205 functions as a buffer layer.
After this, the coated optical fiber 200C comprised of the optical fiber 200A given by the resin coatings 204 and 205 is taken up on a takeup machine 115 through a takeup capstan 114.
The carbon hermetic coating 203 is provided for the purpose of protecting the cladding 202 and improving the mechanical strength and the water resistance, but when there is no such need for this, a coated optical fiber 200C without the carbon hermetic coating 203 in FIG. 2 is used.
The coated optical fiber 200C produced by the method shown in FIG. 2 is formed of a core 201 of about 10 .mu.m at its center, a cladding of a diameter of 125 .mu.m at its periphery, a carbon hermetic coating 203 of a thickness of about 50.ANG. at the periphery of the cladding, and further, for example, a primary resin coating 204 of a thickness of about 10 .mu.m and, for example, a secondary resin coating 205 of a thickness of about 35 .mu.m, thereby giving a coated optical fiber 200C having an outer diameter of about 215 .mu.m overall. This example shows the cross-sectional construction of a coated optical fiber 200C with a carbon hermetic coating 203. Usually, the outer diameter of the coated optical fiber is about 450 .mu.m, but the thicknesses of the core 201 and the cladding 202 do not change, so in this case the thicknesses of the primary resin coating 204 and the secondary resin coating 205 are made greater.
If a solid object touches the surface of the optical fiber 200 right after it is drawn from the optical fiber preform 102, the surface will be damaged. When dust is deposited on it, the mechanical strength of the optical fiber 200 will deteriorate. Therefore, as mentioned above, in the past, the surface of the optical fiber 200 just after drawing or the surface of the optical fiber 200A with the carbon hermetic coating 203 is coated with resin coating layers 204 and 205 comprised of a thermal curable resin, an ultraviolet curable resin, etc.
To improve the production efficiency of the coated optical fiber, it was attempted to raise the drawing speed from, for example, 500 m/minute or so to 1000 m/minute or so. The increase of the drawing speed reduces the manufacturing time and causes a reduction in the manufacturing cost of the coated optical fiber, but the high temperature optical fiber 200 exiting from the drawing furnace 101 or the high temperature optical fiber 200A just after the formation of the carbon hermetic coating 203 in the hot CVD furnace 105 is then supposed to be introduced into the resin coating apparatuses 106 and 110. Therefore, it is necessary to lower the temperature of the optical fiber 200A before introduced into the resin coating apparatus 106.
As the method for this, there are the methods of (1) providing a forced cooling apparatus after the outer diameter measuring unit 104 when not forming a carbon hermetic coating 203 and after the hot CVD furnace when forming the carbon hermetic coating 203 so as to force cool the optical fiber or (2) increasing the distance between the outer diameter measuring unit 104 or the hot CVD furnace 105 and the resin coating apparatus 106 to allow natural cooling of the optical fiber. The provision of the forced cooling apparatus means that the distance between the outer diameter measuring unit 104 or the hot CVD furnace 105 and the resin coating apparatus 106 is increased and the height of the building the optical fiber producing apparatus is raised. Similarly, even in the case of natural cooling, the height of the building for the optical fiber producing apparatus is raised.
The raising of the height of the building the optical fiber producing apparatus is not recommended for economic reasons etc.
As a means of improvement, recently, for example, as disclosed in U.S. Pat. No. 4,510,884, proposal has been made of the method of successively coating the surface of the optical fiber with not only a single resin coating layer, but a multiple resin coating layers. That is, there is known the method of combining the two resin coating apparatuses 106 and 110 shown in FIG. 1 into a single unit and simultaneously coating the resin coatings 204 and 205.
A cross-sectional view of the optical fiber coating apparatus disclosed in U.S. Pat. No. 4,510,884 is shown in FIG. 3A, while a top plane view is shown in FIG. 3B.
In this optical fiber coating apparatus, dies 53 and 63 and a nipple 54, each having a circular outer shape, are housed superposed on each other interspaced with two resin chambers 55 and 65 in a circular-section holding space 52 of a holder 51. Resin supply paths 56 and 66 are connected with the resin chambers 55 and 65. Two resin coating layers are provided about the center of the axis of passage of the optical fiber 57 comprised of a core and cladding (and if necessary further a carbon hermetic coating) running through the holes 53a, 63a, and 54a of the centers of the dies 53 and 63 and the nipple 54. These dies 53 and 63 and nipple 54 are affixed by soldering to the holder 51.
In this optical fiber coating apparatus, if the center axes of the holes 53a, 63a, and 54a of the second layer die 53, the first layer die 63, and the nipple 54 do not match, then the thickness of at least one resin coating layer deposited on the optical fiber or the carbon hermetic coating will not be uniform which will in coated fiber cause the deterioration of the temperature characteristic of the optical fiber.
The reason for this will be explained taking as an example the low temperature attenuation characteristic. When an optical fiber 57 coated with a resin is exposed to low temperatures, the resin covered on the outer surface of the optical fiber 57 will contract. If at that time the resin coating layer is nonconcentric in thickness, the amount of contraction in the outer surface direction will not be uniform. As a result, buckling arising due to the uneven contraction force will occur in the optical fiber 57 and will lead to an increase in the signal transmission loss in the optical fiber 57 due to microbent.
In this way, minute deviations in the center axes of the holes 53a, 63a, and 54a of the dies 53 and 63 and the nipple 54 (hereinafter referred to simply as the "deviations") present extremely significant problems in this type of optical fiber coating apparatus. The reason is that the optical fiber cable, as mentioned above, has the cross-sectional dimensions of a core diameter of about 10 .mu.m, an outer diameter of the cladding of 125 .mu.m, and a primary layer of a thickness of about 10 .mu.m, which requires the sizes of the holes of the dies and the nipple to be small, and it is necessary to align the axial centers very precisely.
Considering the optical fiber coating apparatus disclosed in U.S. Pat. No. 4,510,884 from this viewpoint, since the inside shape of the holding space 52 of the holder 51 and the outer shapes of the dies 53 and 63 and the nipple 54 are all circular, for example, when the hole 54a to be provided at the center of the outer shape of the nipple 54 itself is slightly deviated, correction is required to make the hole 54a be positioned at the center while maintaining the circular outer shape of the nipple 54. In actuality, however, this correction of position is not easy under the dimensional conditions of the optical fiber mentioned above. Therefore, there was the problem that sometimes a new nipple 54 with a corrected position of the hole 54a had to be made. The dies 53 and 63 suffer from the same type of problems as above.